In article , B.B.
says...
Rather than measure everything out and go for exact grinding I just
eyeballed it this first time around. I was also operating under the
assumption I could get the bit as hot as I could stand holding without
doing any damage. Got some blue and brown on it, but otherwise looks as
much like http://www.sherline.com/images/grndfg10.gif as I could
remember at the time.
That one would work fine. You don't really need a chipbreaker for
what you are doing. I respect harold a lot but he's pretty
compulsive about doing things the 100% right way. For small depths
of cut you won't see any real detriment to not having a chipbreaker
except maybe the chip coming off will mark the finish to some
slight degree.
Try grinding a turning tool that is similar to the sherline one,
but don't put any back rake on it - use only three grinds to form
1) front clearance, 2) side rake, and 3) side clearance. It
will look like the one on the far right side of this photo:
http://www.metalworking.com/DropBox/_2000_retired_files/Tp3.jpg
and in that shot the tool would be cutting *away* from the viewer.
Another view of the same tool, where it would be cutting towards the
viewer and slightly to the right:
http://www.metalworking.com/DropBox/_2000_retired_files/Tp2.jpg
And the final view from underneath:
http://www.metalworking.com/DropBox/_2000_retired_files/Tp1.jpg
From the photos you can see it's pretty similar to your sherline
diagram, but the grind for the top surface is parallel to the long
axis of the tool, there's no 'back' angle. The edge thus formed
between the top surface and the side grind is nearly exactly along
the original edge of the side and top of the square cross-section
tool. What this means is you can resharpen this kind of tool
by simply grinding its *front* face, and the height of the
cutting edge does not change.
It's only three grinds on the HSS tool blank - if you do the
front, and side, but no top grind to form side rake, you've
made a brass turning tool.
I've found that back rake has never been essential for the stuff
I do, at work or at home. So I just leave it out.
If you *wanted* to form a chipbreaker in that kind of tool,
the idea would be to run a narrow groove alongside the cutting
edge, on the top surface. This allows the chips to flow off
at a larger angle than the real rake angle, and they eventually
strike the grooves far edge and snap.
When I took a shop class for the first time, there was an entire
four hour class period devoted to trying to get HSS tools ground
properly. Apparently in some classes the instructor simply
hands out 1/4 CRS blanks for the students to practice on.
One final caveat for you: pedestal grinders look pretty innocuous.
They're not. While you are grinding tool blanks by hand, the
blank will become hot, and your hand will become tired. You will
be applying a fair amount of pressure to the tool blank to force it
against the wheel.
The wheel will remove flesh faster than you can imagine - I personally
ground a pretty good-sized divot out of my thumb when I slipped
while grinding a lathe tool. Didn't bleed much at first but that
thing throbbed for weeks afterwards. Take your time and think
of the wheel surface the same way you would if you were grinding
your tool on the roadway, out of an open car door. I bet it's
about the same SFPM.
Jim
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